Semiconductor laser diodes, manufactured as single emitter lasers or laser diode bars, have a high electrical-to-optical conversion efficiency, and can presently achieve optical power levels of a few Watts or even tens of Watts per single emitter laser diode, and tens to hundreds of Watts per laser diode bar. Due to high efficiency, reasonable power levels, and high spectral and directional brightness, laser diodes and laser diode bars find applications in many areas, such as material processing, offset printing, medical treatment, pumping of solid state lasers, and pumping of fiber lasers.
However, light emitted from a high power single emitter diode laser is typically highly asymmetric due to a thin-slab geometry of the laser diodes resulting in long and thin emitting apertures. The light beam emitted by such lasers has much higher brightness in its “fast axis”, which is perpendicular to the main p/n junctions and to the active layer of the laser, than in its “slow axis”, which is parallel to the active layer. Many applications require that the light be coupled into an optical fiber, which generally has a substantially circular or polygonal cross-section and has a substantially symmetrical acceptance angle. To obtain the highest brightness out of the fiber, light beams from multiple single emitter diode lasers are coupled into a single fiber stacked in their fast axis direction, as described for example in U.S. Pat. Nos. 6,898,222, 7,668,214, which are incorporated herein by reference, and US Patent Applications 2009/0245315 and 2009/0323752, which are also incorporated herein by reference. For example, an array of 3-10 individual laser emitters with a 100 micrometers (um) aperture width in the slow axis can be coupled into a fiber with a 105 um diameter and 0.15 NA (numerical aperture) by stacking individual laser beams in the fast axis direction.
Since the diode laser emission is typically polarized, polarization beam combining may be used to couple light emitted by two arrays of single emitters into a single fiber, thereby doubling the power and brightness of the output beam. One example of such laser beam combining device, which incorporates both the spatial stacking of equally polarized laser beams with polarization multiplexing of stacked beams from two laser arrays, is disclosed by Horikawa in U.S. Pat. No. 4,978,197, which is incorporated herein by reference. The beam combining device of Horikawa is a substantially three-dimensional device and is illustrated in FIG. 1. In this device, two rows of lasers 3 are positioned on an upper plate 2A of a multi-level support structure 2. A middle level plate 2B positioned beneath the upper plate 2A includes two rows of collimating lenses 4. The light beams emitted by the lasers 3 downwardly normally to the plates 2A and 2B, are collimated by the lenses 4, and then reflected by two rows of vertically offset prism mirrors 5 positioned on a lower plate 2C, to form two vertically stacked beams that are polarization combined using a polarization beam combiner (PBC) 7 and a half wave plate 9. One drawback of the device of Horikawa is that it is relatively bulky due to, at least in part, its essentially three-dimensional structure, which may also complicated assembly and alignment. Another drawback is that dissipating heat from the laser diodes may be more complicated.
In another example, U.S. Patent Application 2008/0198893 to Bartoschewski et al describes a laser assembly that is illustrated in FIG. 2. In this laser assembly, light from two groups of vertically offset semiconductor lasers 100-106 and 107-113, each of the lasers incorporating a fast-axis collimator, is re-directed by two groups of mirrors 200-206 and 2007-213 along two parallel paths, wherein two slow-axis collimators 301 and 302 are positioned. The slow-axis collimated light beams are then combined using a PBC 400 and a half wave plate 500.
In the laser assembly of FIG. 2, the semiconductor lasers 100-113 in each of the two groups are oriented differently, and so are the reflectors 200-213 associated therewith. This arrangement, although conserving space occupied by the assembly, can disadvantageously complicate the optical alignment of individual elements in the assembly, and its overall fabrication.
An object of the present invention is to overcome at least some of the disadvantages of the prior art by providing a beam combining light source that would be compact, relatively easy to assemble and align, and would utilize polarization beam combining of light from multiple single emitters.